BEIJING, Dec. 29 -- In Yemen, the Arab world's poorest country that is increasingly becoming a hub for Al Qaeda to plot terrorist attacks, the US has secretly opened a third battleground against the terrorist network.

Senator Joseph Lieberman, chairman of the U.S. Homeland Security Committee, said on Fox News Sunday that the U.S. had a "growing presence" in Yemen, including Special Operations, Green Berets and intelligence, according to Reuters.

The U.S. has quietly been supplying military equipment, intelligence and training over the past year to Yemeni forces, who raided suspected Al Qaeda hideouts this month, Reuters cited unnamed U.S. counter-terrorism officials and experts as saying Monday.

The Pentagon is spending more than $70 million over the next 18 months in Yemen, and using teams of Special Forces to train and equip the Yemeni military, Interior Ministry and coastguard forces, more than doubling previous military aid levels, the New York Times reported Sunday.

Yemen came under the spotlight on December 25 when a 23-year-old Nigerian man who tried to blow up an airplane over Detroit claimed that he received training in Yemen and that the explosive device he used was obtained there.

Yemen has long been a base for Al Qaeda militants to launch their jihad against Western targets but was neglected as U.S. and NATO troops concentrated on Afghanistan and Iraq.

Militants bombed the U.S. Navy warship USS Cole in the Yemeni port of Aden in 2000, killing 17 U.S. sailors.

Yemenis were one of the largest groups to receive training in Al Qaeda camps in Afghanistan before the 9/11 attacks. Of the 198 prisoners left at the U.S. military prison at Guantanamo Bay, Cuba, 91 are from Yemen.

Yemen has grown rapidly as a haven for Al Qaeda militants over the past year. Saudi and Yemeni militants joined forces in January this year under the name "Al Qaeda in the Arabian Peninsula," using Yemen as their base.

"An anti-American sentiment has long existed among Muslims in that region because of the perceived U.S. double standards on Palestinians and Israelis, in addition to the confrontation with the Muslim world that resulted from the anti-terrorism policy of George W. Bush," said Li Weijian, an expert on Middle East issues at the Shanghai Institutes for International Studies.

"Not all the radical groups or individuals in Yemen and other countries in that region are affiliated to Al Qaeda," Li said, "But the anti-American sentiment can be easily turned to acts."

Besides combating Al Qaeda militants, the Yemen government has been fighting Shiite Muslim rebels, known as the Houthis, in the north, and faces separatist sentiment in the south.

Fearing instability in Yemen could turn into a security threat for Saudi Arabia, the world's biggest oil exporter, Saudi troops began attacking Yemen's Shiite rebels in early November after the rebels staged a cross-border incursion and killed two Saudi border guards.